1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to, a method for forming a gate oxide film for a high voltage device, and a gate oxide film for a cell and low voltage device in a NAND flash device.
2. Discussion of Related Art
In general, a flash memory device is defined as a high voltage device region where a high voltage device is formed, and a cell region where a cell and a low voltage device are formed. Gate oxide films of transistors formed in the high voltage device region and the cell region are different in thickness according to characteristics of each region. That is, a thickness of a tunnel oxide film formed in the cell region is smaller than that of a gate oxide film formed in the high voltage device region.
FIGS. 1A to 1C are sectional SEM photographs showing a conventional semiconductor substrate where a tunnel oxide film and a gate oxide film have been formed.
Referring to FIGS. 1A to 1C, an element isolation process is performed on the semiconductor substrate where the gate oxide film for the high voltage device has been formed in a high voltage device region and the tunnel oxide film has been formed in a cell and low voltage device region, to form an element isolation film. Here, the element isolation film is protruded from the cell and low voltage device region due to a step difference between the gate oxide film for the high voltage device and the tunnel oxide film. That is, an effective fox height (EFH) of the gate oxide film for the high voltage device is 0, but the EFH of the cell and low voltage device region (protrusion of the element isolation film) is approximately 233 Å.
The high voltage device region is damaged during the succeeding process due to the EFH difference. Generally, the gate oxide film for the high voltage device is formed, a cleaning process is performed thereon, and the tunnel oxide film is formed. Here, the semiconductor substrate is damaged due to an excessive cleaning process, and thus characteristics of the tunnel oxide film are deteriorated.
FIG. 2 is a graph showing constant current stress test (CCST) characteristic results of the tunnel oxide film in the conventional process.
As shown in FIG. 2, the CCST characteristic measurement damages the oxide film and tests a collapse time point of the oxide film. The longer the time from damage to collapse is, the more excellent characteristics the oxide film has. In addition, when variations of the collapse time from the top to bottom are constant, the oxide film is uniformly formed. The CCST characteristics of the tunnel oxide film in the conventional process notify uniformity of the whole oxide film and generation of initial defects.